Hitherto, as the element members of semiconductor devices, photosensitive devices for use in electrophotography, image input line sensors, image pickup devices, or other electric and optical devices, there have been proposed a number of deposited films such as amorphous semiconductor films, for instance, an amorphous deposited film composed of an amorphous silicon material compensated with hydrogen atoms and/or halogen atoms such as fluorine atoms or chlorine atom [hereinafter referred to as "A-Si(H,X)"]. Some of such films have been put to practical use.
Along with those amorphous semiconductor films, there have been proposed various methods for their preparation using plasma chemical vapor deposition technique wherein a raw material is decomposed by subjecting it to the action of an energy of direct current, high frequency or microwave to thereby form a deposited film on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum. And there have been also proposed various apparatuses for practicing such methods.
Now, in recent years, the public attention has been focused on plasma chemical vapor deposition processes by means of microwave glow discharging decomposition [hereinafter expressed by the abbreviation "MW-PCVD process"] also at industrial level.
One representative apparatus for practicing such an MW-PCVD process is a structure as shown in a schematic perspective drawing of FIG. 2(A).
In FIG. 2(A), there are shown a substantially enclosed cylindrical deposition chamber 1 having a raw material gas feeding means (not shown), a microwave introducing window 2 which is made of a dielectric material of alumina ceramics or fused silica, a wave guide 3 being electrically connected to a microwave power source (not shown), microwave 4 from said microwave power source, an exhaust pipe 5 being connected through an exhaust valve (main valve) to an exhaust apparatus (vacuum pump)(not shown), a substrate 6 onto which a deposited film is to be formed, and which is placed on a substrate supporting means having an electric heater (not shown) and a film forming space (plasma generation space) 7 having a resonant structure.
The film forming operation in the apparatus shown in FIG. 2(A) is carried out, for example, in the following way.
That is, the air in the film forming space 7 is evacuated by opening the main valve of the exhaust pipe 5 to bring about the space to a predetermined vacuum. Then the heater installed in the substrate supporting means is actuated to uniformly heat the substrate 6 to a predetermined temperature, and it is kept at that temperature.
At the same time, raw material gases, for instance, SiF.sub.4 gas and H.sub.2 gas in the case of forming an amorphous silicon film, are introduced through the gas feeding means respectively at a predetermined flow rate into the film forming space 7 of the deposition chamber 1 while maintaining the space at a vacuum of less than 1.times.10.sup.-2 Torr. Successively, the microwave 4, for example, of 2.45 GHz from the microwave power source is introduced through an isolator, power monitor, stub tuner (these are not shown) then the wave guide 3 and the microwave introducing window 2 into the film forming space 7 of the deposition chamber 1.
Thus, the plasmas are generated in the film forming space 7 and cause chemical interactions resulting in formation of a deposited film on the substrate 6.
Another representative apparatus for practicing the above MW-PCVD process is a structure as shown in a schematic perspective drawing of FIG. 3(A).
In FIG. 3(A), there are shown a substantially enclosed cylindrical deposition chamber 1, a microwave introducing window 2 which is made of a dielectric material of alumina ceramics or fused silica, a wave guide 3, microwave 4 from a microwave power source (not shown), an exhaust pipe 5 which is connected through a valve means to a vacuum pump (not shown), a substrate 6' in cylindrical form being place on a substrate supporting means having an electric heater 15, film forming space 7 and a gas feeding ring pipe 16 provided with a number of gas liberation holes which is connected to gas reservoirs (not shown).
The film forming operation onto the substrate 6' in cylindrical form using the apparatus shown in FIG. 3(A) is carried out in the same way as mentioned in the case of the apparatus shown in FIG. 2(A).
By the way, in the known apparatus for the formation of a deposited film using MW-PCVD process, said plasmas generated in the film forming space 7 are ionized media comprising electrons and ion particles so that they function as a kind of conductor. Especially, in the case where plasmas are excited with a microwave power of 2.45 GHz, ion particles capable of moving follow with the oscillation having a high frequency are limited to those of a low mass such as electrons. Therefore, in the case of considering the density of the generated plasmas, it will be sufficient to have an attention on the electron density. However, when plasmas generated under such conditions that the vacuum is 2.times.10.sup.-2 Torr and the microwave power is 200 W are such low pressure discharge plasmas as having the electron temperature (T.sub.e) of about 4 electron volt (hereinafter expressed by the abbreviation "eV") and an electron density of n.sub.e =10.sup.17 m.sup.-3, the microwave of 2.45 GHz is reflected at the plasma interface which is about 10 .mu.m distant from the microwave introducing window so that it can not be introduced into plasmas. Because of this, the plasma density becomes decreased abruptly as the distance from the microwave introducing window increases.
In view of the above, in order to form a desired deposited film composed of an A-Si(H,X) material on a large area substrate using microwave plasmas by means of such conventional apparatus as mentioned above, it is necessary to use a microwave introducing window of a large aperture.
In that case, such microwave introducing window is to be disposed to an apparatus so as to serve as a wall of the vacuum chamber 1 in any event, so that the scale of the apparatus inevitably becomes large to invite problems in relation to the strength of the apparatus. Because of this, there will occur a necessity to make a careful consideration in designing the apparatus. In addition, there is also another problem in that the volume of the film forming space 7 becomes large accordingly whereby the utilization efficiency of a raw material gas is reduced. In this regard, even if a desired deposited film product should be produced, it will become costly.